Non-destructive testing of thin anisotropic materials using plane wave excitation and detection has been studied extensively. In recent years, an interest has emerged in the development of dispersion curve inversion and group velocity inversion procedures. The general approach in many of these procedures is to align the excitation event, also referred to as the xe2x80x9ctransduction mechanism,xe2x80x9d and detection location, along the known crystal axis, and to analyze the detected signal to determine elastic constants, bond quality, thickness, or any of a number of additional parameters. Unfortunately, many of these techniques fail to perform adequately when the target""s crystal axis is rotating with respect to the excitation axis, or when the coordinate form of the crystal axis is unknown.
The present invention is directed to a method and apparatus by which an arbitrarily-oriented anisotropic thin material may be interrogated for characterizing an unknown material property value thereof The material property may comprise, for example, temperature, hardness, elastic constants, composition, crystal orientation, grain size, pressure, and residual stress, or any material property that is variable with respect to known material physical parameters.
For purposes of the present invention, the known material physical parameters may comprise a subset of the unknown material property values. To distinguish them, the unknown material property values to be characterized are referred to herein as xe2x80x9cmaterial property values,xe2x80x9d while the known material property values are referred to herein as xe2x80x9cmaterial physical parameters.xe2x80x9d
In a first embodiment, the method of the present invention is directed to a method for generating theoretical functions to characterize an unknown material property value of a thin anisotropic material. First, a model of the thin material is generated. The model preferably comprises the behavior of known material physical parameters, for example elastic constants and material density, as functions of the material property value to be characterized, for example temperature. For a plurality of known material thicknesses and known material property values, a transduction mechanism is simulated at a source location for generating a simulated elastic stress wave operating on the model at a plurality of source locations. The simulated intensities of signals generated by the simulated elastic stress waves are computed at a sense location to provide a representative composite signal. Theoretical functions, for example a matrix of equivalent modal excitation functions, are determined for symmetric modes and anti-symmetric modes from the composite signal at each thickness and at each material property value.
In a preferred embodiment, following determination of the theoretical functions, an elastic stress wave is generated in a material of unknown thickness and unknown material property value at a source location. The intensity of a measured signal generated by the elastic stress wave is sensed at a sense location positioned a known distance from the source location. The measured signal is correlated to the theoretical functions to determine correlation values. The material thickness value and material property value are determined based on the best correlation values.
In a preferred embodiment, the step of correlating comprises pattern recognition, or alternatively a variation of in-step correlation. The transduction mechanism may comprise a simulation of ring excitation comprising a plurality of line excitations arranged tangentially about a ring. The material property preferably comprises a property selected from the group consisting of temperature, pressure, elastic constants, density, hardness, composition, crystal orientation, grain size, and residual stress.
In a second aspect, the present invention is directed to a method for empirical characterization of a transduction event in a material. A transduction event is initiated in a material to generate a measured signal. An initial estimate of the transduction event is propagated along known dispersion curves characterizing the material to generate a theoretical signal. The measured and theoretical signals are decomposed and their respective amplitudes are determined along the dispersion curves. In a preferred embodiment, the average amplitudes are determined. The decomposed measured and theoretical signals are compared to generate an error signal. The error signal, in turn, is used to modify the initial estimate.
In a preferred embodiment, the steps of propagating, decomposing, comparing, and modifying are performed iteratively until the error signal is within acceptable limits. The theoretical signal is preferably time-shifted with respected to the measured signal such that they are substantially contemporaneous in time. The characterized transduction event is preferably of the form of a modal excitation function. The material is preferably a thin anisotropic material which may be of unknown thickness. The steps of propagating and temporal-decomposing are preferably performed along a plurality of unique orientations for the material. The step of temporal decomposing may comprise for example decomposing using the continuous wavelet transform, the short-time Fourier transform, the Wigner-Ville transform, and/or the Choi-Williams transform. The measured and theoretical signal may each comprise symmetric and anti-symmetric modes of propagation.